Intraspecific Competition

Intraspecific Competition

“competition is an interaction between individuals, brought about by a shared requirement for a resource in limited supply, and leading to a reduction in the survivorship, growth and/or reproduction of the competing individuals concerned”

Begon, et al. Chapter 6

Two types of competition

• Scramble (exploitative) competition

* no direct interaction

• Contest (direct or interference) competition

* some type of confrontation

Four characteristics of intraspecific

competition• The ultimate effect: decreased contribution of individuals to the next generations

• The resource must be in limited supply.

• Competing individuals are all essentially equivalent.

• The effect on any individual increases with increasing number of competitors

Density dependent mortality

a=density independent; b=undercompensating density dependance; c=overcompensating density dependent

Exactly compensating density-dependent

mortality

Mortality/fecundity equilibrium

Reality means there is a broader range in which an equilibrium can be found

Law of Constant Final Yield

• At low densities yield increases with density

• Eventually yield becomes independent of density

L, M, N = nutrient levels

Total yield

Root wt.

Shoot wt.

Carrot Density and

Yield

Timing and Size

Plants emerging later grow less than predicted by average wt. gain/day

Dactylis

Asymmetric

Competition

Density & time of emergence both play a role

Flax

Results of Intraspecific competition

• Stress

• Dispersal

• Social interactions

* dominance

* territoriality

Territories• Type of territory

* general - breaks down after breeding season

* nesting - (feeding is done elsewhere)

* food resource

• Means of “defending”* Sound* Visual

* Scent marking

Example: Red Grouse

• Three social classes* territorial cocks with hens* nonterritorial surplus birds - on periphery

* nonterritorial transient birds

• Fall to Spring - territories defended* By late winter, all surplus birds removed* # & size of territories determined by amount of heather & N content of foliage

Red grouse breeding & survival

Time Event

August

September

October

November

January

April

Strife - family breakup

Displaced birds

Nonterritorial transientsClass 3

Class 3 birds do not breed

Most Class 3 birds dead

Territorial behavior(young and old) Successful territorial

defenders

Territorial ownersClass 1High survivalreplacement

Resident surplus birdsClass 2

Move to marginal habitat

Fail to breedSurvival poor

Family groups(territorial behavior byold cocks at dawn)

Breeding stock

When is defense profitable?

• When competition exists• When enough resources exist• When costs do not outweigh benefits

Convict Cichlid Fish• As territory size increases:* Chase rate* Growth rate

Territory size vs. animal size